Method of manufacturing cold-rolled nonoriented electro-6 magnetic steel sheet and product electromagnetic steel sheet

ABSTRACT

IN THE METHOD OF MANUFACTURING A COLD-ROLLED NONORIENTED ELECTROMAGNETIC STEEL SHEET COMPRISING SI IN AN AMOUNT OF 1.0-4.0% BY WEIGHT, ACID SOLUBLE AL IN AN AMOUND OF 0.1-3.0% BY WEIGHT AND THE REST BEING IRON, COMPRISING THE ORDINARY PRODUCTION STEPS OF DECARBURIZATION, AT LEAST ONE COLD-ROLLING STEP, AND A FINAL HEAT-TREATMENT SUBSEQUENT TO THE FINAL COLD-ROLLING STEP (INCLUDING SKIN PASS OR TEMPER ROLLING), THE IMPROVEMENT WHICH COMPRISES THE DECARBURIZATION TREATMENT BEING CARRIED OUT IN THE PRODUCTION STEPS BEFORE THE FINAL COLD-ROLLING STEP, THE INNER OXIDIZED LAYER PRESENT IN THE SURFACE PART OF THE STEEL BEING REMOVED AFTER THE DECARBURIZATION TREATMENT BUT BEFORE THE FINAL COLD-ROLLING STEP; ALL THE HEAT-TREATMENTS AFTER THE REMOVAL OF THE INNER OXIDIZED LAYER BEING CARRIED OUT IN A NON-OXIDIZING ATMOSPHERE WHICH CONTAINS WATER AND HYDROGEN IN SUCH AMOUNTS THAT THE RATIO OF THE PARTIAL PRESSURE OF STEAM FORMED DURING THE HEATTREATMENT STEPS TO THE PARTIAL PRESSURE OF HYDROGEN IS LESS THAN 0.05 TO AVOID THE RE-FORMING OF AN INNER OXIDIZED LAYER.

Sept. 10, 1974 |ZUM| sus ETAL 3,834,952

METHOD OF MANUFACTURING COLDROLLED NONORIENTED ELECTED-6 MAGNETIC STEEL SHEET AND PRODUCT ELECTROMAGNETIC STEEL SHEET Filed March 29, 1971 4 Sheets-Sheet 1 FIG. 6

(Weft/kg) FIG. I

Fe. SiO2.A|2Oa Sept. 10, 1974 z Mi s s n ETAL 3,834,952

METHOD OF MANUFACTURING COLD-ROLLED NONORIENTED ELECTRO-G MAGNETIC STEEL SHEET AND PRODUCT ELECTROMAGNETIC s'rEEL SHEET Filed March 29, 1971 4 Sheets-Sheet a FIG. 2 FIG. 4

MAGNIFICATION 1 Q 6 MAGNIFICATIONI Q VERT 5000x,HOR IOOx VERT5000x,HOR IOOx FIG. 3 FIG. 5

OXIDI- LAYER BASE IRON MAGNIFICATION I MAGNIFICATION I IINAER ZED VERTSOOO, HOR IOOX VERT 5000. HOR IOO x p 10, 1974 IZUMI MATSUSHITA ETAL METHOD OF MANUFACTURING COLD -ROLLED NONORIE-NTE ELECTHO-6 MAGNETIC STEEL SHEET AND PRODUCT ELECTROMAGNETIC STEEL SHEET 4 Sheets-Sheet 5 Filed March 29, 1971 may,

ATTORNEYS I 3,834,952 Patented Sept. 10, 1974 United States PatehtQfice 3,834,952 METHOD OF MANUFACTURING COLD-ROLLED NONORIENTED ELECTRO-6 MAGNETIC STEEL SHEET AND PRODUCT ELECTROMAGNETIC STEEL SHEET Izumi Matsushita, Kiyoshi Tanada, and Osamu Honjo, Himji, Japan, assignors to Nippon Steel Corporation, Tokyo, Japan Filed Mar. 29, 1971, Ser. No. 128,673 Claims priority, application Japan, Mar. 30, 1970, 45/ 26,295 Int. Cl. H011? N04 US. Cl. 148-112 20 Claims ABSTRACT OF THE DISCLOSURE In the method of manufacturing a cold-rolled nonoriented electromagnetic steel sheet comprising Si in an amount of 1.04.0% by weight, acid soluble Al in an amount of 0.1-3.0% by weight and the rest being iron, comprising the ordinary production steps of decarburization, at least one cold-rolling step, and a final heat-treatment subsequent to the final cold-rolling step (including skin pass or temper rolling), the improvement which comprises the decarburization treatment being carried out in the production steps before the final cold-rolling step, the inner oxidized layer present in the surface part of the steel being removed after the decarburization treatment but before the final cold-rolling step; all the heat-treatments after the removal of the inner oxidized layer being carried out in a non-oxidizing atmosphere which contains water and hydrogen in such amounts that the ratio of the partial pressure of steam formed during the heattreatment steps to the partial pressure of hydrogen is less than 0.05 to avoid the re-forming of an inner oxidized layer.

BACKGROUND OF THE INVENTION Field of the Invention facturing a cold-rolled nonoriented electromagnetic steel sheet excellent in characteristics at high magnetic field intensity and electromagnetic steel sheet products.

Description of the Prior Art Electromagnetic steel sheets have been previously called silicon sheets or electric steel sheets and are widely used to form iron cores for electrical machinery and apparatus. There are a variety of electromagnetic steel sheets classified according to the various rolling methods, rolling orientations, configurations and other factors, such as hot-rolled silicon steel sheets, hot-rolled and welded nonoriented silicon steel sheets or strips, cold-rolled nonoriented silicon steel sheets or strips, and cold-rolled oriented, or anisotropic, silicon steel sheets or strips, which materials are all widely known:

Generally, these electromagnetic steel sheets are required to have various characteristics, among which the magnetic characteristics in particular should be excellent and which sheets are graded in terms of iron loss.

In order to improve the magnetic characteristics, the iron loss must be reduced. Since the iron loss consists of the hysteresis loss and eddy current loss, it is necessary should possess a preferred orientation suitable for exhibiting magnetic characteristics.

On the other hand, in order to effect a decrease in eddy current loss, it is necessary to coat the steel sheet with an electric insulation film so that, when the steel sheet thus coated is used as material for a laminated iron core, the steel sheet components of the core are electrically insulated. Besides, the steel sheet must contain alloying elements which heighten the electrical resistance of steel sheet itself, such as silicon (Si) and aluminum (Al), and further the thickness of the steel sheet should be decreased.

For the production of a cold-rolled nonoriented electromagnetic steel sheet, which is a type of electromagnetic steel sheet, manufacturing processes have been devised in view of the above described factors as a matter of course so as to achieve the desired magnetic characteristics. For example, a well known fundamental method for producing cold-rolled nonoriented electromagnetic steel sheets employs the so-called one-step cold-rolling method in which hot-rolled coils formed by the rolling of slabs containing not more than 4.0% Si, 0.1 to 3.0% acid soluble aluminum (Sol. Al) and as small a per centage of other impurities as possible are annealed according to the necessity, cold-rolled to the desired sheet thickness and thereafter subjected to bright continuous annealing or box annealing. Besides, where it is intended to obtain higher-class electromagnetic steel sheets containing larger crystal grains so as to have a lower iron loss, there is another conventional manufacturing method available which employs the two-step cold rolling method (the single cold rolling and skin pass rolling method) wherein the above-mentioned hot-rolled coils, after annealed as required, are cold-rolled to the desired thickness at two stages with an intermediate annealing stage in between and thereafter subjected to a bright continuous annealing or a box annealing. The cold-rolled nonoriented "electromagnetic steel sheet manufactured by any of the above described prior methods fails to have any desirable iron loss value particularly at a high magnetic field intensity. This is related to the fact that the said steel sheet is graded in accordance with the Japanese Industrial Standards (JIS). However the iron loss values adopted for this grading are specified with regard to the products by the aforesaid conventional methods, so that W10/50 (the iron loss at a magnetic flux density of 10 kg. and a frequency of 50 Hz.) is the proof value while WIS/50 (the iron loss at magnetic flux density of 15 kg. and a frequency of 50 Hz.), that is, the value at a higher field intensity is the reference value. At present, these iron loss values are set upon the basis of lower magnetic flux densities than those when the said electromagnetic steel sheet is put to practical use to form the iron cores for electrical machinery and apparatus, and particularly for rotary machines.

As a result, the prior cold-rolled nonoriented electromagnetic steel sheets have drawbacks in that the iron loss increases markedly as the magnetic flux density heightens when the steel sheets are used as materials for iron cores in electrical machinery, especially for rotating machines. More particularly, the relation between magnetic flux "density and iron loss is such that the iron loss is increased with increasing flux density. This phenomenon is especially remarkable when the flux density is above 15 kg. for this reason, it has so far been extremely diflicult to manufacture desirable cold-rolled nonoriented electromagnetic steel sheet suited to form iron cores for rotary electric machinery.

SUMMARY OF THE INVENTION object of this invention is to provide cold-rolled nonoriented electromagnetic steel sheets with markedly reduced iron loss during heightened magnetic field intensity, i of thecarbon in electromagnetic steel sheet products, the by improving the conventional methods ef-manufaeturing content being preferably below 0.005%. A method gencold-rolled nonoriented electromagnetic steel sheets, and to provide an advantageous method for manufacturing such novel steel sheets.

Another object of the present invention is to providea' method for manufacturing a cold-rolled'nonoriented electromagnetic steel sheet excellent in characteristics athigh magnetic field intensity by improving'the prior methods for producing cold-rolled nonoriented electromagnetic steel sheets, the improvement being applicable with effortless ease to any of the aforesaid prior basic methods so that the iron loss under a strong magnetic field is markedly lower.

These objects of the present invention are achieved by providing a method for manufacturing a cold-rolled nonoriented electromagnetic steel sheet excellent'in characteristics at high magnetic field intensity which includes a decarburizing stage, a final cold rolling and a final heat treatment so as to produce a cold-rolled nonoriented electromagnetic steel sheet containing 1.0 to 4.0% silicon (Si), 0.1 to 3.0% acid-soluble aluminum (Sol. Al) and as small a percentage of other impurities as possible, which method is characterized by the improvement as providing the decarburizing treatment before at least one combination of cold rolling and heat treatment, removing the inner oxi dized layer extending in the surface portion of the steel sheet after said decarburizing treatment and prior to the cold rolling, and, after removing the inner oxidized layer, performing the heat treatment in an atmosphere wherein the partial pressure of steam divided by that of hydrogen is kept below 0.05 to prevent the formation of any inner oxidized layer.

BRIEF DESCRIPTION OF THE DRAWINGS Various other objects, features and attendant advantages of this invention will be more fully appreciated as the same becomes better understood from the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic sketch of the oxidized layers formed in the surface portion of the steel sheet at. the hot rolling stage of the present invention.

corresponding to the sketch of FIG. 1.

FIG. 3 is a photograph illustrating a cross-sectional aspect immediately beneath the surface of a conventional electromagnetic steel sheet.

FIGS. 4 and 5 are respectively photographs showing cross-sectional aspect beneath the surface of electromagnetic steel sheets embodying the present invention.

FIG. 6 is a graph for comparison of iron'loss curves of an electromagnetic steel sheet having an inner oxidized erally adopted to lower the carbon content in the product electromagnetic steel sheet is to provide a decarburizing annealing stage adequately following the hot-rolling opera tion, Generally employed as such a decarburizing anneal ing method is the atmosphere decarburizing method in which annealing is effected in the so-called wet atmosphere which is composed of several tens percent hydrogen and the remaining percentage of nitrogen and steam with a dew point of several tens of degrees C. Besides, the scale decarburizing method which makes use of the mill scale of hot-rolled coils, or the vacuum decarburizing and refining method applicable to molten steel is now practically employed.

In the aforesaid hot rolling, the atmosphere is wet air having a strong oxidizing property, so that an outer oxidized layer (0) consisting, as shown in the sketch of FIG. 1 and the photograph of FIG. 2, of Fe O Fe O- FeO, $0, and 2FeOSiO for example, that is, a layer of scale is formed on the hot-rolled coil surface, while an inner oxidized layer (1) containing SiO A1 0 and iron oxides is formed beneath the outer oxidized layer (0) on the surface portion of the base iron. The outer oxidized layer (O), or the scale, can be removed by means of such an ordinary pickling solution as a dilute solution of sulfuric acid or hydrochloric acid, whereas the inner oxidized layer (I) cannot be removed with such a solution.

Furthermore, the atmosphere for the decarburizing annealing operation, after the hot rolling, has a weak oxidizing quality as well as a decarburizing property. Thus, even if the surface of the decarburized steel sheet is bright, the decarburization is accompanied by the formation of the above described inner oxidized layer, as shown in FIG. 3, in the surface portion of the cold-rolled steel sheet which has no outer oxidized layer, that is, no scale layer. The inner oxidized layer formed in this case is usually as thick as several microns to several tens of microns, and cannot be removed by means of an ordinary pickling solution, just as in the aforesaid case.

After making various studies with a view to improve the characteristics at high magnetic field intensity of a cold-rolled nonoriented electromagnetic steel sheet, the FIG. 2 1s a photograph showing a cross-sectional aspect present inventors have ascertained that the iron loss at high magnetic field intensity can be markedly decreased by removing the above described inner oxidized layer.

As an illustration of this fact, an example is cited hereinafter. A cold-rolled nonoriented electromagnetic steel layer at each side and an electromagnetic steel sheet deprived of inner oxidized layers from both sides. I FIG. 7 is a graph showing the relationship between the thickness of the inner oxidized layer and the iron loss.

FIGS. 8 and 9(A) (B) are respectively work process diagrams based on the method of the invention.

DESCRIPTION on THE- PREFERRED EMBODIMENTS The'present invention will be more fully understood by reference to the following detailed description ofthe 'prerecrystallized grains, or the carbon existing in the "forms of impurities and deposits in the'steel, 'de'teriorate'the magnetic characteristics. Further, the-carbon in'the's'tel sheet fosters the penomenon of magnetic aging wheh'the sheet is used to form an iron core, thus causing the iron loss to increase. Therefore, in order to prevent magnetic aging in particular, it is necessaryto lower thegcontent sheet (A) having an inner oxidized layer of 5 microns in thickness and a final thickness of 0.35 mm. was produced by pickling a hot-rolled coil containing 0.025% C, 3.1% Si, 0.003% and 0.26% Sol. Al by a known method so as to remove the scale and practicing the two-step coldrolling method (the single cold rolling and skin pass rolling method) With the use of a wet atmosphere for the intermediate annealing and the final annealing. Meanwhile, another cold-rolled nonoriented electromagnetic steel sheet (B) was manufactured by pickling the same hot-rolled coil having undergone the scale removal and the intermediate annealing, in a pickling solution composed of 6% nitric acid, 1% hydrofluoric acid and the remaining percentage of distilled water, thereby removing the inner oxidized layer, and thereafter carrying out the skin pass rolling to a final thickness of 0.35 mm., and the subsequent final annealing operation with the use of an annealing atmosphere wherein the PH O/PH (the partial pressure of the steam in the atmosphere divided bythat'of the hydrogen therein) was 0.04. The relationship between the magnetic flux density (MF) and iron 1655 (WL) of each of the steel sheets (A) and (B) thus produced is shown in FIG. 6. It will be apparent from FIG. 6 that, with the final product (B) deprived of the inner oxidized layer, the iron loss (WL) is remarkably lowered at high field intensity values. I

A further illustration of the invention is as follows: A material C (hot-rolled coil) containing 2.1% Si and 0.23% Sol.Al, and a material D (hot-rolled coil) containing 2.9% Si and 0.25% 801. Al, and a material E (hot-rolled coil) containing-3.0% Si and 0.39% Sol. Al werepiokled', thereby being deprived of the respective scale. Thereafter, the materials were cold-rolled to an intermediate sheet thickness and subjected to decarburizing annealing to a carbon content below 0.005% in a wet atmosphere composed of 20% hydrogen and the remaining percentage of nitrogen and steam (with a dew point of 35 C). The three materials thus annealed were deprived of their inner oxidized layers in different amounts by the use of the same pickling solution as in the preceding'example and by changing the pickling time. Then, the materials were subjected to skin pass rolling, which is cold rolling with a low reduction ratio, to a final thickness of 0.35 mm, and thereafter finally annealed in.a:dry atmosphere (the dew point being 5 C.) composed of 20% hydrogen and the remaining percentage .of nitrogen. The thickness of the inner oxidized layer (T(,u)) of each of the three cold-rolled nonoriented electromagnetic steel (C), (D) and (E) thusmanufactured and the iron loss (WL (W17/50) at a high magnetic fiux density (17 kg.) are related as shown in FIG. 7. As will be apparent from FIG. 7, the iron loss varies according to the material constituents, and the higher the Si content and the S01. Al content, the lower the iron loss, while the iron loss at the high field strength decreases markedly with the reduction in the thickness of the inner oxidized layer.

The mechanism by which the inner oxidized layer alfects the iron loss has not yet been clarified in detail. However, it iscertain that the iron oxides and alumina, silica and othermetal oxides formed in the inner oxidized layer increase thehysteresis loss by the same action as crystal grain domains, impurity inclusions and other similar inner defects, with the result that the iron loss is increased. This effect of the inner oxidized layer is considered to be particularly remarkable with a strong magnetic field where the magnetic flux tends to concentrate at the surface portion of the steel sheet. In addition, from the fact that the iron loss at a high field intensity is markedly lowered by removing the inner oxidized layer, it can be taken as a matter of course that the iron layer is lowered at a high frequency, by which the magnetic flux is concentrated at the surface portion of the steel sheet to a larger extent than by high field intensity. As will be understood from the foregoing description, the inner oxidized layer existing in the surface portion of cold-rolled nonoriented electromagnetic steel sheet has a great influence upon the iron loss at a high magnetic field intensity, and accordingly the removal of such an inner oxidized layer is the primary feature of the present invention.

The pickling prior to the cold rolling of hot-rolled coils is known in the art. The pickling solution is normally a dilute solution of sulfuric or hydrochloric acid. Moreover, the object of the coventional pickling is to remove the so-called scale composed of iron oxides (Fe O Fe O FeO, Fe SiO thus not to remove the inner oxidized layer, as in the present invention. In fact, the inner oxidized layer cannot be removed at all by means of such acid solutions as have been generally applied. Moreover, conventional acid solutions includes pickling depressors for preventing the dissolution of steel sheet (base iron).

' v0n the other hand, to remove the inner oxidized layer advantages of the present according to the invention, it is essential to employ a means for removing the base iron. For example, a pickling solution capable of positively dissolving the base iron, an electropolishing method, a mechanical removing method employing sand brushing or the like, or any other suitable means is adequately adopted for the said purpose.

In connection with the pickling of hot-rolled silicon steel sheet, there is known, for example, themethod disclosed in the report entitled Removal of Oxide Films of Silicon Steel Sheet and Its Effects Journal of the Institute of Electrical Engineers of Japan, Vol. 61, No.

641, December 1941. However, the oxide films dealt with in this report are, as specified therein, films separable during the production of machines and apparatus with the use of the steel sheet and films which can be easily rubbed off with a piece of polishing paper and can be removed also by means of a pair of tweezers, that is, they are identical with the iron oxide layer (scale) formed during hot rolling, thus being entirely different from the inner oxidized layer to be removed by the present invention. Furthermore, such inner oxidized layer cannot be removed by the use of the dilute sulfuric acid (at a temperature of 60 to C.) having a concentration of about 10% which is introduced in the report, as will be clear from the foregoing description. The inner oxidized layer is not separated even by a high degree of cold rolling. Thus, the above cited report does not suggest anything concerning the removal of the inner oxidized layer.

The secondary feature of the present invention lies in specifying the stage of removing the inner oxidized layer and the decarburizing stage and also employing a specific atmosphere for the heat treatment following the stage of removing the inner oxidized layer, in the one-step coldrolling method or the two-step cold-rolling method (the single rolling and skin pass rolling method) employed in conventional methods for manufacturing cold-rolled nonoriented electromagnetic steel sheet. Where the onestep cold-rolling method or the two-step cold-rolling method is to be practiced under conventional concepts, it will normally be considered to provide the stage of removing the inner oxidized layer after the completion of the final heat treatment. However, if the inner oxidized layer is removed after the final heat treatment, some drawbacks will appear. That is, when pickling or electropolishing is carried out under such severe conditions as removing the inner oxidized layer in a short time in the order of a few seconds or minutes, then, because the dissolving (polishing) speed varies according to the orientation of the crystal grains composing the steel sheet and because the steel sheet contemplated in the invention is a high-class electromagnetic steel sheet with low iron loss which generally has coarse crystal grains, the steel sheet surface becomes uneven and also tends to have corrosion holes formed therein. As a result, when the steel sheet is used to form an iron core, the space factor is lowered, with the appearance. impaired. On the other hand, when a mechanical polishing method based on sand brushing or the like is employed, internal stresses act near the surface of the steel sheet, and therefore the iron loss is heightened and also streak flaws are formed on the steel sheet surface, impairing the appearance of the steel sheet in many cases.

Various studies made be the inventors have revealed that these demerits can be overcome by providing such improvement as comprises, in the one-step cold-rolling method or the two-step cold-rolling method (the single cold rolling and skin pass rolling method), removing the inner oxidized layer after the decarburizing treatment and, at the latest, before the final cold rolling and heat treatment, performing cold-rolling thereafter, and carrying out the subsequent heat treatment under such conditions that no inner oxidized layer can be formed, that is, in an atmosphere wherein the PH O/PH (the partial pressure of the steam by that of the hydrogen in the atmosphere) is not more than 0.05.

That is to say, as the removal of the inner oxidized layer which has been formed by the hot-rolling and decarburization annealing, is carried out before the final recrystallization annealing, the crystal size of the layer is still not large at the time of removal, nor is the roughness of the surface of the steel sheet very pronounced whichphenomenon may be caused by the chemical removal'.ofsthe=layerzAlsozas the cold-rolling-step is provided afterthe removal of said inner'oxidized layer, such roughness and scratches that may be produced by the mechanical removal of the layer, can be eliminated by coldrolling; and the inside stress produced by the mechanical removal can be eflFectively put off by the heat treatment following the subsequent cold-rolling. However, if the atmosphere in which the heat treatment is carried out after the removal of the inner oxidized layer is an oxidizingatmosphere, such layer will be again produced. To avoid reforming this oxidizing layer, the heat treatment after the removal of the inner oxidized layer is carried out in an atmosphere of less than 0.05 in PH O/PH Thus, there is obtained a cold-rolled, non-oriented electrolayer, and therefore has h gh magnetic field properties magnetic steel sheet which is free from the inner oxidized and a good surface.

Based on the above explanation, we can present Processes (1)-(I5) as nine preferable processes of the present invention, provided, however, the present invention is not limited to these processes.

FIG. 8 shows a process diagram for the method of the present invention practised with the one-step cold rolling method.

FIG. 9(A), (B) shows a process diagram for the method of the present invention practised with the twostep cold-rolling method.

The explanation of the processes shown in FIG. 8 is as follows.

In order to meet conditions such that the cold-rolling step (4) and the heat treatment step (5) are required as the step subsequent to that for the removal of the inner oxidized layer and pickling step (3), it is necessary to remove the inner oxidized layer :as well as the surface scale by using a pickling solution which is so strong as to dissolve the base iron, at the inner oxidized layer removing and pickling step (3) before cold-rolling step (4). In this case, decarburization is carried out in the form of a hot rolled coil by the decarburizing annealing step (2) as shown in the process (1) or the vacuum degassing step (6) by the vacuum degassing treatment at the stage of molten steel as shown in the process (II). The atmosphere in which the final heat treatment step (5) is carried out is less than 0.05 in PH O/PH The explanation of the processes shown in FIGS. 9(A) and 9(B) is as follows:

The process (III) is represented by: Hot-rolling step (1)-pickling step (7) (for removing -seale)primary cold-rolling step (8)--intermediate heat-treatment and decarburizing annealing step (9)the inner oxidized layer jrernoval step '(10)-second'ary cold-rolling step (11) skin pass or temper rolling)final heat-treatment step (12) (dry annealing).

The processes (IV), '(V) and (VI) are featured by: Hot-rolling step (1) and the hot-rolled coil decarburizing annealing step (2), respectively; and the processes (VII), (VIII) and (IX) are featured by the vacuum degassing step (6) through the vacuum degassing treatment of molten steel and hot-rolling step (1), respectively. Any v of the above-mentioned processes (IV), (V), (VI), (VII), (VIII) and (IX) may be represented as follows: The processes (1V) and (VII) involve the removal of the inner. oxidized layer as well as scale by pickling step (Bl-primary cold-rolling step (8)-intermediate heat- Itreatrnent step ('13),-(dry. annealing)secondary coldrolling step '(11) (skin pass or temper rolling)-heattreatmentstep (12) (dry annealing); the processes (V) andi(VIII)-involve the pickling step (7) (for removing scale)primary cold-rolling step (8)the inner oxidized layer removal-step '(10)-intermediate heat-treatment step. (13). (dry annealing)=second-ary cold-rolling step (11) -(slin' pass or temper ro1ling)final heat-treatment 'step-(-12)' (dry annealing); and the processes (VI) and (IX) involve the pickling step (17 :(t'or remevingsgtilea primary cold-rolling step (8)int ermediate;heat:treat ment step 14) (wet annealing)f(ordry anneaiing -the inner oxidized layer removal-step (10.)*seconjdary-j'eold'-. rolling step (11) (skin pass or temper rolling'yand'the final heat-treatment step(12): (.dry annealing). .1

As is clear from the above explanation, theme odof the present invention is so constructed that-an inner oxidized layer removal step is provided leaving'atleas-t one set of a cold-rolling and a heat treatmentsteg'mnd a decarburizing step is provided as a step-previoustoz-said inner oxidized layer removal step; and the heatctreat'ment subsequent to said inner oxidized layer removal rstep'ris carried out in an atmosphere incapable ofproducin'g an inner oxidized layer. l I f T The reason why Si is limited 'to between 1.0;-4.0% according to the present invention is that a steel sheet of less than 1.0% Si content is a low-grade product which does not especially require any iron loss(w+att loss) ."in the high magnetic field intensity, therefore, it is -outs ide'th scope of the present invention; and a steel sh'eet of more than 4.0% Si content is very brittle, making-it difficult to subject it to cold-rolling without encounteringdi flicultiesl On the other hand, Sol. Al is limited to 0.-l'+3 0% The reason is that the S01. Al content of less than 0.1% makes ditficult the recrystallization attlie heat-treatment sub'se quent to the cold-rolling, and 801.. Al of more-than 3.0% makes the steel sheet brittle, in combination with the -Si content, making it diflicult to subject it to'cold-roilin'g Without encountering ditliculties In order to make the'ele'ctromagnetic steel'shee't' 'noriaging magnetically, }it is desirable to 're'duce the carbon content to less than 0.005% by decarburizationf' As mentioned above, the atmosphere -at the heat treat ment subsequent to the removal ofthe inneroxidized layer is limited to less than 0.05 in; PH O/PH FH thej partial pressure ratio is more than 0.05, an inneroxidi'zed layer forms rapidly, making itimposs'ible to achieve the object of the present invention. 3 Regarding the removal of said inner 'oxidized'l'aye'r according to the present invention, the reductionbf'iron loss which is one of the objects of the "present invention; can be achieved, even if such removal is not perfectfbut leaves an inner oxidized layer in a thickness of ash s As shown in the photo of FIG. 5, that is, in theicas'e of FIG. 5, the iron loss is W /50 3.56 watt/kg. after removal (W /50 3.43 watt/kg. after the perfect removal of inner oxidized layer 0 which has been lowered so sufficiently from the iron loss of W 50" 4.09 watt/kg. (thickness of inner oxide layer 4 before removal to nearly meet the purpose of removal. Therefore, according'to the present invent-ion, he un removed inner oxidized layer is permitted, provided that it is less than 1p. thick, because such thickness does not prevent the reduction of the iron loss. 7 i f The following are examples of the presentinvritiori; Having generally described the invention, the'follo'wing specific examples are provided for purposesofillustration only and are not intended to be limiting unless 'ot he'i wise specified.

Example 1 (the one-step cold-rollingmethod) t Hot rolled coil annealing was carried out at750"C. for 8 hours with a hot rolled-coil of '20 mm. "thick-and containing carbon 0.035%=-by weight,'silicon='2.3%'iby weight, Sol. A1 0.15% by weight and the: restbeing essentially iron with mill scale, so as to decarburize'whe carbon content to 0.0032%. Then,'*the followihgtwo methods were applied to the sample tobe'reducd -to the final sheet product of 0.50 mm; thick.

A (the conventional method): Afte'r'being pickled with a pickling solution consisting of 2%'hydrochlori'c ac"id and the rest being water to remove scale, the sample was reduced to the final product of 0.50 mm; thick by cold 9 rolling. Then, this'was annealed at 925 C. for 3 minutes in, a dry atmosphere consisting of 40% hydrogen and the rest being nitrogen-(Dew point: C.; PHgO/PH =0.015). B (the present invention, the process I shown in FIG. 8): After being pickled with a pickling solution consist- 10 A (the conventional method): After being reduced to an intermediate sheet of 0.39 mm. thick by primary coldrolling, the sample was subjectedto intermediate annealing in an atmosphere consisting of 20% hydrogen and the rest being nitrogen, and reduced to a final product (D) present invention ing" of 2% hydrochloric acid, 3% nitric acid, 1% hydroof 0.35 mm. thick by secondary cold-rolling and then gen fluoride and the rest beingwater in the inner oxidized annealed at 850 C. for 4 minutes in the same atmosphere layer removal and pickling step (3), to remove scale and as was used in the method above. theinne'r oxidized layer, the-sample was reduced to a B the present invention, the process VI, shown in final product of 0.50 mm. thick in the cold-rolling step FIG. 9(B)): After the intermediate heat treatment step (4). Then, thiswas annealed in the same atmosphere as (14) (annealing) of the same treatment as was used in wasu'sed in the methodA above, at 900 C. for 3 minutes the method A above, the sample was removed by the inth'e final heat treatment step (5). inner oxidized layer removal step (10) with a chemical The steel sheet samples respectively obtained by the polishing solution consisting of 6% hydrogen peroxide, above-mentioned methods were cut into Epstein test 2% hydrogen fluoride and the rest being water, and then pieces; and the results of tests are shown in Table 1. reduced to the final sheet product of 0.35 mm. thick by secondary cold-rolling step (11) of the same as was used Example 2 (the two-step cold rolling met od) in the method A above, and annealed at 850 C. for 4 fA being Pickled With Pickling Solution consistminutes in a dry atmosphere consisting of 25% hydrogen ing of 2% hy rochloric acid n he r ing r, and the rest being nitrogen (PH O/PH :0.023) in the to m v S 1 ,a- 0 611 Coil f thick, confinal heat treatment step (12). The cross-section of the 'taining carbon- 0.029% by weight, silicon 3.1 by, so-obtained steel sheet is shown microscope-photographweight, Sol. Al 0.17% by weight and the rest being ically in FIG, 4. essentially iron, was reduced to an intermediate sheet C (the present invention, or process V, shown in FIG. of 0.39' mm. thick by primary cold-rolling, and Was Sub- 9 (B) After the primary cold-rolling step (8), the same iected t0 decarburizing annealing to decarburizfi the as used in method A above, the steel sheet was pickled bon content to 0.0035 in a wet atmosphere consisting ith a pickling solution prepared by adding 3% hydrogen of 25% hydrogen and the rest being nitrogen and steam fluoride to concentrated hydrochloric acid in the inner (D Point! n the sample was treated re oxidized layer removal step (10), the removal of the spectivly y' following tWO methods- 3 inner oxidized layer, subjected to intermediate annealing ("thnnonventional method) {After being ce to in the dry atmosphere consisting of 25% hydrogen and the a P Product of thlck y y rest being nitrogen (PH O/PH =O.05) in the intermediate f i the t gl p i' ni f 2%; -h 4 Inlmlte; heat-treatment step (13), and then reduced to the final m ry h are cohsls lhg O 0 y rogeh ah sheet product of 0.35 mm. thick in the secondary coldl l ezt fr ai n i gri t mtrogen (PH2O/PH2=0'02) m the final 35 rolling step (11) and annealed at 850 C. for 4 minutes B (the present invention as exemplified by the process in g h g 2 lattlmosphere as was used In the above III shown in FIG. 9(A)): After the surface was ground met 0 mt 6 na trhfatment Step (12) with asandbrush, in-the inner oxidized layer removal D (the present i or process ih step (10), the sample was subjected to the same secondary 40 9'(A) The Plckhhg hhhseqheht the dhcarbhnzlhg 'cold rolling Step (11) and finalheamreatment Step (12) annealing of hot rolled C011 was carried out 1n the same was carried out themethod A above manner as mentroned above but with a pickling solution Thevsteel sheet-samplesfobtained respectively by the consisting of 2% hydrochloric acid, 3% nitric acid, 1% above-mentioned methods were cut into Epstein test hydrogen fluoride and the Test being Water in the inner pie e jand the re ult are ho in Tabl 1, 4.5 oxidized layer removal pickling step (3), to remove the Example 3 (the two step cold roumg method) annealing product and the inner oxidized layer. The r 1 3 so-treated sample was reduced to the intermediate sheet After being pickled w th a pickling solution conslstlng f 039 thi k in the primary cold-rolling step (8), 2% hydrochlonc ac1d h the rest e Water to and subjected to intermediate annealing in the dry atmosremove scale, .a hot rolled C011 of the same kind as was phere consisting of 25% hydrogen and the rest being used in Example 2 above, was formed into a loose coll, nitro en (PH ()/PH 025) in the inter t h t and was subjected to hot rolled coil decarburizing annealt t 2 13 2 h d d fi 8 5 ing at 800 C; for 7 hoursinthe strongly decarburizing, rea ment 5 ep an t en re Hoe to nal S eet wet atmosphere consisting of 88% hydrogen and 12% of i thlck m the Secqndary coldofolhng step. (11) nitrlvogenvand Steam (Dew point: 539 O), to decarburize and sub ected to final annealing at 850 C. for 4 Irnnutes the-carbon content, to 0.0027%, and then again acid- In the same dry atmosphere as was used above In the washed with the pickling solution of the same kind as final heat treatment step (12)- wasused The samples obtained respectively by the above four Th Sample was t t d ti l b th f ll i methods were cut into Epstein test pieces, and the results 4.'methods: of tests on these pieces are shown in Table 1.

TABLE 1 Thickness or'inner oxidized Iron loss, watt/kg. Magnetic flux layer density, kg. unremoved Number Example method (1*) W /m W15/50 W /so B25 B50 1 (A) conventional 5. 6 1. 72 3. 84 5.78 15. 4 16. 3 I p (B) present invention 0.8 1.69 3.68 4.95 15.5 16.4

2 (A) conventional 4.7 1.06 2.69 4.16 15.2 16.1 (B) present invention 0.5 1.02 2.45 I 3.21 15.8 r 16.2 a (A) conventional 3.3 1.02 2.56 4.09 15.3 1 16.2 (B) present invention 0. 4 0.98 2. 43 3. 22 15. 4 16. 3 (0) present invention 0. 3 0. 97 2. 39 3. 16 15. 4 16. 3 0. 5 2. 41 3. 23 15.4, 16. 3

The smoothness of the steel sheet obtained by the method of the present invention is excellent, therefore, its commercial value and the space factor are high.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit and scope of the claims.

What is claimed is: A

1. A method of manufacturing a cold-rolled non-oriented electromagnetic steel sheet which has a greatly reduced iron loss when subjected to a high magnetic field, said steel sheet containing Si in an amount of 1.0-4.0% by weight, acid-soluble Al in an amount of 0.-13.0% by weight and the rest being substantially iron, which comprises:

-(a) hot-rolling the steel material to form a steel sheet in an atmosphere of wet air having strong oxidizing properties,

(b) annealing the steel sheet to decarburize it in an atmosphere having weak oxidizing properties,

() removing the inner oxidized layer formed in the surface of the steel sheet, which layer contains SiO A1 0 and oxides of iron to such an extent that it is present in an amount less than 111. in thickness,

(d) cold-rolling the steel sheet, and

(e) heat-treating the steel sheet in a non-oxidizing atmosphere which contains water and hydrogen in such amounts that the ratio of the partial pressure of steam formed druing the heat-treating step to the partial pressure of hydrogen is less than 0.05 to avoid the re-forming of an inner oxidized layer.

2. The method according to claim 1, wherein the removal of said inner oxidized layer is carried out by pickling the steel sheet with a pickling solution consisting of hydrochloric acid 2%, nitric acid 3%, hydrogen fluoride 1% and the rest being water.

3. The method according to claim 1, wherein the removal of said inner oxidized layer is carried out by electrolytic polishing with a chemical polishing solution consisting of hydrogen peroxide 6%, hydrogen fluoride 2% and the rest being water.

4. The method according to claim 1, wherein the removal of said inner oxidized layer is carried out by pickling the steel sheet subsequent to the primary cold-rolling with a pickling solution made by adding 3% hydrogen fluoride to concentrated hydrochloric acid.

5. The method according to claim 1, wherein the removal of the inner oxidized layer is carried out by use of an electrolytic polishing method.

6. The method according to claim 1, wherein the removal of the inner oxidized layer is carried out by use of a sand brushing method.

7. A method according to claim 1 wherein the steel sheet is pickled with a strong acid, capable of etching the iron in the steel sheet, to remove both the outer oxidized layer and the inner oxidized layer.

8. A process according to claim 1 wherein the inner oxidized layer is completely removed.

9. A cold-rolled non-oriented electromagnetic steel sheet of low iron loss at high magnetic field intensity consisting essentially of Si in'an amount of 1.0-4.0% by weight and acid-soluble Al in an amount of 0.l3.0% by weight, the rest being substantially iron, having an inner oxidized layer of less than 1, in thickness produced by the process of claim 1.

10. A method of manufacturing a cold-rolled, nonoriented electromagnetic steel sheet which has a greatly reduced iron loss when subjected to a high magnetic field, said steel sheet containing Si in an amount of 1.0-4.0% by weight, acid soluble Al in an amount of (Ll-3.0% by weight and the rest being substantially iron, which comprises:

(a) vacuum-degassing the steel material in the molten form to decarburize it,

(b) hot-rolling the material to form a steel sheet in an 12 atmosphere of wet air having strong oxidizing proper? ties, I Y (c) removing the inner oxidized layer formed in the surface part of the steel sheet, which layer contains SiO A1 0 and oxides of iron to such an extent that it is presentin an amount less than i in thickness, I (d) cold-rolling the steel sheet, and V (e) heat-treating the steel sheet ina non-oxidizing atmopshere which contains water and hydrogen in such amounts that the ratio of the partial pressure of steam formed during the heat-treating stepto the partial pressure of hydrogen is less than 0,05 to avoid the re-forming of an inner oxidized layer.

11. A process according to claim 10 wherein the inner oxidized layer is completely removed.

12. A method of manufacturing a cold-rolled, non-'- oriented electromagnetic steel sheet which has a greatly reduced iron loss when subjected to a high mangetic field, said steel sheet containing Si in an amount of l.0-'-4.0% by weight, acid-soluble Al in an amount of (kl-3.0% by weight and the rest being substantially iron, which comprises: w

(a) hot-rolling the steel toform a sheet in an atmosphere of wet air having strong oxidizing properties,

*(b) annealing the steel sheet to decarburize it m an atmosphere having weak oxidizing properties,

(c) subjecting the steel sheet to a first cold-rolling (d) removing the inner oxidized layer formedin the surface part of the steel sheet, which layer-contains SiO A1 0 and oxides of iron to such an extent that it is present in an amount less than 1p. in thickness, (e) subjecting the steel sheet to a second annealing step in an atmosphere having weak oxidizing properties, vi

(f) subjecting the steel sheet to a'second cold-rolling step, and

(g) heat-treating the steel sheet in a non-oxidizing atmosphere which contains water and hydrogen in such amounts that the ratio of the partial pressure of steam formed during the heat-treating step to the partial pressure of hydrogen is less than 0.05 to avoid the re-forming of an inner oxidized layer.

13. The method according to claim 12, wherein said decarburization annealing and said second annealing are carried our simultaneously between the first cold-rolling step and said removal of the inner oxidized layer.

14. The method'according to claim 12, wherein the removal of the inner oxidized layer is carried out between the decarburization annealing and said first cold-rolling 15. The method according to claim 12, wherein the secondary annealing step is carried out between the first cold-rolling and the removal of the inner oxidized layer.

16. A process according to claim 12 wherein the inner oxidized layer is completely removed.

17. A method of manufacturing a cold-rolled, 'non oriented electromagnetic steel'sheet which'ha's' a greatly reduced iron loss when subjected to a high magnetic'fie'ld, said steel sheet containing Si in an amount of 1.0-4.0% by weight, acid-soluble .Al in an amount of 0.1-3.0% by weight and the rest being substantially iron, which comprises:

(a) vacuum-degassing the steel material in the molten state to decarburizeit,

=(b) hot-rolling the steel to form a sheet in an atmosphere of wet air having strong oxidizing properties,

-(c) removing the inner oxidized layer formed in the surface of the steel sheet, which layer contains 810:, A1 0 and oxides of iron to such an extent that it is present in an amount less than about 1, in thickness, and oxide of iron,

(d) subjecting the steel sheet to a first cold-rolling step,

(e) subjecting the steel sheet to an annealing step in an atmosphere having weak oxidizing properties,

(f) subjecting the steel sheet to a second cold-rolling step, and

(g) heat-treating the steel sheet in a non-oxidizing atmosphere which contains water and hydrogen in such amounts that the ratio of the partial pressure of steam formed during the heat-treating step to the partial pressure of hydrogen is less than 0.05 to avoid the reforming of an inner oxidized layer.

18. The method according to claim 17, wherein the removal of the inner oxidized layer is carried out between the first cold-rolling and the annealing step (e).

19. The method according to claim 17, wherein the removal of the inner oxidized layer is carried out between said annealing step (e) and the second cold-rolling step.

20. A process according to claim 17 wherein the inner oxidized layer is completely removed.

References Cited UNITED STATES PATENTS Carpenter et a1. 148-113 X Takahashi 148-1-13 Knuppel et a1 148-1 11 Wiener 148111 Gimigliano 148-113 X Schneider et a1. 148--111 Carpenter et a1. 148-111 Campbell et a1 148-110 Grenoble 148111 Wawrousek 148111 FOREIGN PATENTS US. Cl. X.R. 

